The present invention relates generally to power conversion apparatus and more particularly to a method of developing a linear relationship between the load voltage of a controlled converter bridge and a command which determines that load voltage.
It is common practice in the discipline of power conversion to use a bridge arrangement of controllable devices, such as thyristors or transistors, to provide an output voltage of controllable magnitude and/or frequency. Such bridge arrangements are most commonly used to convert alternating current (AC) electrical power to direct current (DC) electrical power or DC power to AC power, although similar arrangements for AC to AC and DC to DC are also known. In these converters, the several controllable elements are sequentially rendered conductive in some predetermined order to effect the desired output of the converter.
Perhaps the most common form of converter in use today, at least in the higher power fields, is the thyristor bridge converter. Employing well known phase control techniques, thyristor converters are able to provide electrical power outputs of controllable magnitude and/or frequency. An ideal control for such a converter would be one which causes the load voltage to be linearly proportional to a command voltage. One problem associated with these bridge converters, as well as some other types, is that they tend to exhibit a non-linear relationship between their load voltage and the command (a command voltage or reference).
The most common technique in use today for adjusting for this non-linearity is through the use of "biased cosine" firing logic, an example for which is described in Chapter 10 of Thyristor Phase-Controlled Converters and Cycloconverters by Brian R. Pelly - Wiley Interscience, Copyright 1971, Library of Congress Catalog Card Number 70-125276 which publication is specifically incorporated hereinto by reference. This biased cosine firing technique, although not one hundred percent effective performs adequately for most purposes so long as the converter output current remains continuous. It has been found, however, that when the current becomes discontinuous, the non-linearity between the load voltage and the command voltage or reference substantially increases.